Field of the Invention
This invention relates to an electrophotographic printing method and related apparatus for forming a toner image on a photosensitive medium. More particularly, it relates to a reversal imaging method and related apparatus for fixing toner particles on a portion of a photosensitive medium which is exposed to a light beam selectively projected thereon in accordance with an image of an object, to form a toner image of the object for subsequent printing on a suitable recording medium (e.g., paper).
There have been developed various electrophotographic printers in which a latent electrostatic image is formed by projecting an optical beam onto a photoconductive layer of a photosensitive medium. The resultant latent electrostatic image is thereafter developed into a toner image by depositing toner particles on the photoconductive layer. The toner image thereafter is transferred onto a recording paper and fixed thereon.
The principle of one such prior art method is illustrated in FIGS. 1(a) to 1(c), to which the following description is directed. A photosensitive medium 1 comprises an electrode 7 and a photoconductive layer 8, such as a selenium layer which may be evaporated on the electrode 7. Initially, the medium 1 is charged uniformly by a corona discharging device 2, as shown in FIG. 1(a), to produce a uniform layer of ions, illustrated as positive ions in the example of FIG. 1(a). Subsequently, an optical beam, such as a laser beam, is projected in the direction designated by the arrow L onto the surface of the photoconductive layer 8, for selectively exposing same and rendering the exposed portions conductive, thereby selectively discharging the positive ions thereon to ground. The optical beam is scanned over the photoconductive layer 8 and its optical density is controlled, in accordance with an image to be printed. The latent electrostatic image thus formed then is developed to form a toner image, by using a magnetic brush developer 4 as illustrated in FIG. 1(c). For this purpose, the electrode 7 is grounded and a positive voltage is applied to the developer 4, wherein fine particles referred to as toner particles 6 are mixed with relatively coarse iron particles, referred to as carriers 5. The toner particles 6 are charged triboelectrically, and adhere to the surface of the photosensitive medium 1 in a pattern corresponding to the latent electrostatic image, as shown in FIG. 1(c). The visual toner image, thus produced on the photosensitive medium 1, subsequently is transferred to and fixed on a suitable recording paper (not shown).
The use of a corona discharging device for charging the photosensitive medium layer in such prior art electrophotographic printing systems, as described above, presents many problems. For example, to generate the corona discharge, a high voltage source must be provided, typically of several thousand volts (KV). Moreover, the corona discharge is very sensitive to atmospheric conditions, such as the level of humidity and the presence of dust and other contaminants in the air. Additionally, the corona discharge generates ozone, recognized to constitute a health hazard to operators. Thus, corona discharging devices are not only expensive, but present problems of unstable printing operations as well as health hazards to operators. Accordingly, there has been a need in the art to develop electrophotographic printing techniques which eliminate the use of corona discharging devices.
One example of an electrophotographic method which eliminates the use of corona discharging devices is disclosed in Japanese patent application by Ishihara et al., laid open under Provisional Publication No. 119375/82 on July 24, 1982. FIG. 2 is a schematic cross-sectional view of equipment operating in accordance with that method, and serves to illustrate the principle of the method. A photosensitive medium 15 comprises a laminant of a transparent supporting layer 11, a transparent electrode 12 made of ITO (Indium-Tin-Oxide), a photoconductive layer 13 made of Cds (cadmium sulfide), and an insulating layer 14, as an example.
Power source 18 applies a voltage between the transparent electrode 12 and a developer device 17, which may comprise a conventional magnetic brush developer. The developer 17 applies a uniform layer of magnetic toner particles 16 onto the surface of the insulating layer 14. A light represented by the arrow L then is projected onto the bottom surface of the supporting layer 11, for selectively exposing the photoconductive layer 13 in accordance with an image to be produced and rendering those exposed portions of the photoconductive layer 14 conductive. As a result, negative charges 20 are injected into the exposed portions of the photoconductive layer 13 and travel therethrough to the boundary or interface between the photoconductive layer 13 and the insulating layer 14. Positive charges injected into the conductive toner particles 16, meanwhile, are developed at the surface of the insulating layer 14, as illustrated by the positive charges 19 in the toner particles 16 at the surface of the insulating layer 14. The positive charges 19 and negative charges 20 generate a relatively strong electric field across the insulating layer 14. As a result, the particles 16 having the positive charges 19 adhere strongly to the surface of the medium 15, and thus of the insulating layer 14, in those portions thereof which correspond to the exposed areas of the photoconductive layer 13, after extinguishing the scanning light beam L.
Conversely, at those portions of the upper surface of the photosensitive medium 15 which have not been exposed, there is no such strong adhering effect between the toner particles 16 and the insulating layer 14, because the photoconductive layer 13 remains nonconductive in those nonexposed portions and, moreover, because it has a substantial thickness. Accordingly, the toner particles 16 in these nonexposed areas do not adhere strongly to the surface of the insulating layer 14 and accordingly are collected and removed by the developer 17. The retained, adhered particles 16 thus form the toner image.
While the electrophotographic printing method as described in relation to FIG. 2 thus avoids the defects and problems of employing high voltage corona discharging devices, it introduces other problems. For example, the photoconductive layer 13 must be relatively thick to achieve a satisfactory contrast in the toner image, since the formation of the toner image is achieved by utilizing the difference in the adhering force, known as a Coulomb force, generated by each of the respective electric fields, as between the exposed and unexposed regions. The fabrication of such a thick photoconductive layer of uniform thickness is both difficult and expensive, in view of the material costs. Moreover, while increasing the thickness of the photoconductive layer is important to achieve improved contrast, increasing the thickness reduces the photosensitivity of the layer with the resultant requirement of increasing the recording voltage. Moreover, since conductive toner particles are employed in this method, plain paper, in view of its relatively low resistivity, is not suitable as the recording medium to which the toner image is transferred and instead a specially treated medium, for example, a paper coated with an insulating layer, must be used.
As a result, there remains a need in the art for improved techniques of electrophotographic printing which avoid the foregoing and further problems and deficiencies of prior art techniques.